[Konstantinos]

Actually the problem is here: "\n\n\" - you don't need the last character "\".

pawn Код:

ShowPlayerDialog(playerid,dregister,DIALOG_STYLE_MSGBOX,""COL_CYAN"Server",""COL_YELLOW"Welcome to StuntCity Ultimate!\n\n"COL_RED"Please ensure that you got our modification installed,\n\ 
    otherwise you will not be able to stunt or see our specially designed vehicles!\n(Visit http://www.stuntcity.cu.cc for more information)\n\n"COL_YELLOW"\n\nTo continue to the class selection,close the dialog box,and hit the (>>) button on the bottom of your screen."COL_RED,"","OK");